1. Field of the Invention
The present invention relates to accelerometer (G sensor) responsive control circuits which are used to control the operation of devices such as those disposed in automotive vehicles and the like, and more specifically to such types of control circuits which require an accurate prediction of the displacement of an object under the influence of the acceleration in order to effectively implement operation. More specifically, the present invention is concerned with air-bag inflator ignition timing control circuits of a type which predict a displacement of a driver's head by processing an acceleration (or deceleration) of the vehicle and determine the ignition timing of the air-bag inflator in accordance with predicted displacement of the driver's head.
2. Description of the Prior Art
Currently, there is a demand to improve the control of the vehicular related systems with a view of improving occupant safety. One aspect of these endeavors comes in the provision of passenger restraint systems which are designed to improve passenger safety during collisions and like types of accidents.
One type of restraint system takes the form of an inflatable cushioning arrangement generally referred to as an "air-bag". However, air-bags are such as require a finite time to reach maximum inflation and then quickly deflate. Accordingly, it is very difficult to time the detonation of a gas producing charge or the opening of a valve via which compressed gas is released, in order to inflate the bag in a manner which optimizes the cushioning effect of the same. That is to say, in the case of an air bag which is arranged to protect a driver from impact with the vehicle steering wheel, if the inflation is induced too late the inflation degree will be less than optimal at the time the person actually comes into contact with the air bag. This of course increases the chances of injury as a result of impacting on the steering wheel rim or hub. On the other hand, if the timing is too early, the bag will be in the process of deflating when the driver comes into contact with the air bag. This again reduces the cushioning effect of the air bag.
A number of control circuits have been proposed for use with G sensors for timing the detonation of an air-bag inflating charge and/or control of the like type of passenger restraint devices.
One such control circuit is disclosed in U.S. Pat. No. 4,985,835 granted Jan. 15, 1991. This patent discloses a method of determining the time when an automotive safety system, such as an air-bag or the like, should be released or activated. That is, in this method, the displacement of a driver from the seated position at a predetermined time in the future is calculated and predicted based on signals issued from an acceleration sensor, and the safety system (viz., air-bag) is activated when the predicted displacement of the driver exceeds a predetermined threshold value. For the prediction of the driver's displacement, the following formula is used: ##EQU1## wherein: tx=time x
Z=activation time of the safety device PA1 a(t)=acceleration sensor signal at time t PA1 Vrel=relative velocity, calculated by integration of a(t) PA1 SI(t)=projected position of the passenger at time t (calculated by doubled integration of the acceleration sensor signal) PA1 K1 to K4=evaluation factors PA1 integrating an acceleration signal a(t) from the sensor and producing a signal V(t) indicative of velocity; PA1 integrating the velocity indicative signal V(t) to produce a first signal x(t) indicative of displacement; PA1 weighting the acceleration indicative signal a(t) with a factor t.sup.2 d/2 to produce a second signal {1/2.times.a(t).times.t.sup.2 d} indicative of displacement; PA1 weighting the velocity indicative signal V(t) with a time factor (td) to produce a third signal {V(t).times.td} indicative of displacement; PA1 adding the first, second and third displacement signals together to derive a total displacement signal {x(t+td)}; PA1 comparing the total displacement signal with a reference value; and PA1 issuing the first air-bag deploying signal in the event that the total displacement signal exceeds the reference value.
As seen, in the formula, the acceleration signal is subjected to double integration, integration and differentiation for prediction of the driver's displacement upon a vehicle collision. In fact, the formula is provided on the assumption that upon a vehicle collision, the driver would act as a freely movable mass in relation to the vehicle.